1. FIELD OF THE INVENTION
The present invention relates to a multilayer optical interference filter and its use with light transmissive articles including electric lamps, and, more particularly, to a multilayer optical interference filter having a controlled degree of diffuse reflection with respect to a desired visible wavelength range and its use with light transmissive articles including electric lamps, reflectors and/or lenses.
2. BACKGROUND OF THE ART
Thin film optical interference coatings known as optical interference filters are well known to those skilled in the art and comprise alternating layers of two or more materials of different refractive index. Such coatings are used to selectively reflect and/or transmit light radiation from various portions of the electromagnetic spectrum.
These coatings have been found to be useful to improve the illumination efficacy of incandescent and arc lamps by reflecting infrared radiation emitted back to the filament or arc while transmitting the visible light. This lowers the amount of electrical energy required to be supplied to the filament or arc to maintain its operating temperature.
Such films have also been applied to reflectors in the form of what is known in the art as "cold mirrors" and "hot mirrors". A "cold mirror" is an optical filter which reflects visible light while at the same time permitting longer wavelength infrared energy to pass through the filter. This ensures that the light reflected is much cooler than it would otherwise be if both visible and infrared light were reflected. On the other hand, some reflectors contain a reflecting coating on the inside reflecting surface, such as a coating of aluminum, for reflecting essentially all of the radiation emitted by a lamp filament or arc and projecting it forward of the reflector. In this latter case, the projected light is significantly hotter than that obtained with a cold mirror. Finally, a "hot mirror" is an optical filter which reflects infrared radiation while at the same time permitting shorter wavelength visible light to pass through the filter. One application of hot mirrors in the lamp industry has been to return infrared heat to the filament to increase lamp efficiency.
It is often desirable to diffusely scatter light from an intense source, such as an incandescent filament, by providing the envelope of the incandescent lamp with a light diffusing coating. In reflector lamps provided with lenses or reflectors, stippling has been employed on the lenses and facets have been provided on the reflector in order to diffusely reflect the projected filament images.
One such light diffusive coating is disclosed in U.S. Pat. No. 4,869,927 to Kawakatsu et al., the disclosure of which is incorporated herein by reference. A dip-coating method was used to form a light diffusive coating on a halogen lamp envelope by applying an organo-metallic compound combined with a high boiling point solvent to a base and baking so that the organo-metallic compound decomposes to form a metallic oxide coating including bubbles which scatter light formed by the vaporization of the high boiling point solvent. Such a layer may be used alone or included in a multilayer coating consisting of alternating high refractive index layers made of, for example, titania, and low refractive index layers made of, for example, silica.
U.S. Pat. No. 4,663,557 to Martin, Jr. et al., the disclosure of which is herein incorporated by reference, teaches that an 1100.degree. C. post-deposition heat treatment can transform a specularly reflecting tantala/silica optical interference coating provided on a halogen lamp envelope by vacuum deposition such that it will diffusely reflect visible light while continuing to specularly reflect infrared radiation.
It is therefore an object of the present invention to provide a multilayer optical interference filter having a controlled degree of diffuse reflection with respect to a desired visible wavelength range.
It is another object of the present invention to provide an article which is vitreous and light transmissive with a multilayer optical interference filter having a controlled degree of diffuse reflection over a desired visible wavelength range, such as a reflector or lens, useful, for example, with an electric lamp.
It is yet another object of the present invention to provide an electric lamp including a multilayer optical interference filter having a controlled degree of diffuse reflection over a desired visible wavelength range provided on its lamp envelope and/or on a reflector and/or lens associated with the lamp.